Previously known density meters have frequently employed a vibratory tube through which the fluid whose density is to be measured is passed. The vibratory tube has been arranged to vibrate in the circumferential mode of vibration, also known as the bell mode or the hoop mode, and the value of the vibrations has been used to determine the value of the density of the fluid. Such density meters, however, are limited to use on low viscosity fluids, e.g. fluids having viscosities below 20 cS. Vibratory tubes which are arranged to vibrate in the circumferential mode of vibration, moreover, are expensive to make since their manufacture involves precision machining.
Density meters are also known whose vibratory tubes are arranged to vibrate in the clamped-clamped flexural mode of vibration in which the fluid viscosity has very little damping effect on the vibration. However, in previously known density meters employing vibratory tubes which vibrate in the clamped-clamped flexural mode of vibration, complicated structures have been used to balance the reactions at the nodes of the vibratory tubes. Thus in one construction, shown in British Patent Specification No. 1,158,790, this balancing is effected by the use of twin vibratory tubes which communicate with each other adjacent the nodes and which need to match each other extremely closely if the meter is to operate satisfactorily.
In another construction, shown in British Patent Specification No. 1,432,165, only one single vibratory tube is employed, but a mass is connected to the vibratory tube at each node thereof by way of a cantilever system which balances the reaction at the respective node. Such a construction, however, involves complicated matching at one frequency only.